This invention is directed toward the field of furnace controls, and more specifically to the field of trial timers and controllers.
Standard furnace systems usually included a solenoid operated valve (SOV) to control gas flow into a combustion chamber. To control the operation of the SOV, a microcontroller was often used in a furnace control system so that the SOV may be opened and closed at appropriate times.
One time when it was desirable to open the SOV was during ignition of the furnace, also known as a trial. If the furnace did not ignite within a preselected amount of time, usually four or six seconds after the start of the trial, it was desirable to end the trial and close the SOV. Well known flame sense circuitry was included in the furnace control system to sense flame during a trial, and to keep the SOV open once flame was sensed.
Yet, flame was not always created during a trial, and it was left to the microcontroller to end a trial and close the SOV in such a case. As was stated earlier, most furnace manufacturers specified either a four or six second trial time for their furnaces. Thus, a manufacturer of microcontrollers for furnaces had to create a microcontroller which could handle either trial time period. Generally, this resulted in a microcontroller which had two pins, one which would be connected to get a four second trial and one for the six second trial.
Problems were created in that to meet certain certification requirements, any failure of the microcontroller had to lead to the shorter or four second trial time period. Failure which led to a six second trial time period could have caused a serious explosion which could lead to personal injury and property damage.
Lastly, only a limited number of pins were available on the microcontroller.
Thus, it is an object of the present invention to provide a furnace control which fails to a four second trial time. It is a further object of the present invention to provide a furnace control which only requires one pin on the microcontroller to control the trial time period.